In recent years, Orthogonal Frequency Division Multiplexing (OFDM) technique has become a mainstream technique of radio communication physical layer technology because of its effective counteract multipath interference and narrowband interference and high frequency spectrum efficiency, while the technique of Orthogonal Frequency Division Multiple Access (OFDMA)+Multiple Input Multiple Output (MIMO) has the natural technical advantages compared with the 3rd generation Code Division Multiple Access (CDMA), and is more applicable for broadband mobile communication system, and thus is generally acknowledged to be one of the core techniques of the next generation mobile communication system. Worldwide Interoperability for Microwave Access (WiMAX) which adopts OFDMA is the core technique of physical layer and takes account of WiMAX 802.16e standard with both mobility and broadband characteristics, and it is a powerful competitor for the next generation mobile communication standards.
In an OFDMA system, the time domain is divided into a plurality of OFDM symbols, while the frequency domain is divided into a plurality of subchannels and each subchannel is an assemble of a group of sub-carriers. Usually a time-frequency zone crossly constituted by a subchannel and one or more symbols is called a slot, which is the smallest distribution unit in an OFDMA system. So the physical layer resource of an OFDMA frame can be logically denoted by a two-dimensional rectangular table composed of slots and subchannels. As shown in FIG. 1, each grid is a slot, and the physical layer resource allocated for the terminal is a resource block with slot as the unit, and the resource block is usually similar to a rectangular block (e.g. IEEE 802.16e), which is a kind of two-dimensional time-frequency structure.
The time-frequency two-dimensional resource brings various benefits to the OFDMA system, and one of the peculiar advantages is that the terminal reversion has subchannelization gain, which can improve the reverse gain of an OFDMA terminal. As shown in FIG. 2, taking the reverse frame structure of WiMax IEEE802.16e system Fast Fourier Transformation (FFT) 1024-point under the Partial Usage of Subchannels (PUSC) mode for example, the present invention illustrates the concept of subchannelization gain: with 35 subchannels in total, the transmit power of a Mobile Station (MS) which is also called a terminal (suppose that the maximum power Pmax is 23 dBm) can be completely distributed to subchannels that are in practical use, and an extreme case is that Pmax (23 dBm) is completely distributed to one subchannel (the maximum transmit power of each sub-carrier is 9 dBm), and there is a gain of 15.4 dB compared with the situation when a MS occupies all the 35 uplink subchannels.
The WiMax IEEE802.16e system can support both reverse subchannelization and repetition at the same time, and when the transmit power of the terminal reaches the maximum transmit power Pmax, the integrated gain after the process of subchannelization and then repetition is not necessarily equal to the sum of the two gains directly, and the integrated gain may even be less than the gain after using only subchannelization under certain particular circumstances, leading to a sudden deterioration of reverse coverage. Two scenarios will be illustrated hereinafter:
as shown in FIG. 2, suppose that the PUSC mode is adopted, Time Division Duplex (TDD) is 2:1, the proportion of uplink symbol number to downlink symbol number is DL:UL=31:15, each uplink subchannel actually occupies a width of 5 slots, including data and overhead channels; ideal repetition gains for 2, 4 and 6 times of repetition each are 3 dB, 6 dB and 7.8 dB respectively; one-subchannelization gains in terms of 2, 4 and 6 subchannels each are 3 dB, 6 dB and 7.8 dB respectively.
Scenario 1:
when a terminal user is successfully accessed, the service occupies exactly one whole subchannel (4 slots), the user moves from a near point to a distant point until the power reaches Pmax(23 dBm/QPSK), meanwhile, suppose that the terminal continues to move away from the base station, if repetition is activated, for 2 times, the terminal will occupy 2 subchannels, and the gain brought by repetition will be 3 dB under the ideal condition; meanwhile, however, the power of each subchannel will decrease 3 dB, and the transmit power of each subchannel will be 20 dB, so the gain of each subchannel may decrease 3 dB because of the power reduction, therefore, the integrated gain is 0 dB. In other words, it appears that the repetition brings no gain under such a condition. In the same way, 4 times or 6 times of repetition brings no gain either.
Scenario 2:
when a terminal user is successfully accessed, the service occupies exactly the first slot of a subchannel, this user moves from a near point to a distant point until the power reaches Pmax (23 dbm/QPSK), wherein, suppose that this terminal will continue to move away from the base station, if repetition is activated, for 2 times, the terminal will occupy 2 slots, and the gain brought by repetition will be 3 dB under the ideal condition; thereby, the terminal will occupy 2 slots because of the two repetitions, but still occupy 1 subchannel, the power of which is Pmax (23 dbm), therefore, the integrated gain brought by the repetition is 3 dB, in the same way, for 4 times of repetition the terminal also occupies 1 subchannel, with an integrated gain of 6 dB; but for 6 times of repetition, the terminal will occupy 2 subchannels, and a gain loss of 3 dB will be brought by the power reduction, so the integrated gain is 7.8−3=4.8 dB.
It can be seen from the description of scenario 1 and scenario 2 that, under the condition that the transmit power of the terminal is limited, the effect of enabled repetition gets worse under certain particular circumstances.